Beta < + >-gamma-gamma coincidence positron annihilation lifetime-momentum correlation spectrometer

Abstract

The invention discloses a beta < + >-gamma-gamma coincidence positron annihilation lifetime-momentum correlation spectrometer, and belongs to the technical field of nuclear detection. The spectrometer comprises a positron annihilation life detection module, a positron annihilation gamma photon energy detection module, a signal acquisition module and a control and analysis module, and the positron annihilation life detection module is used for acquiring a life part in an AMOC spectrum; the positron annihilation gamma photon energy detection module is used for acquiring a momentum part in an AMOC spectrum by detecting gamma photons generated by positron annihilation; the signal acquisition module is used for acquiring signals generated by the positron annihilation life detection module and the positron annihilation gamma photon energy detection module; the control and analysis module is used for controlling the signal acquisition module and obtaining an AMOC spectrum based on signals acquired by the signal acquisition module. According to the method, the coincidence counting rate can be effectively improved, good coincidence time resolution and extremely low source components are achieved, and the construction cost of a spectrometer can be reduced.

Description

Technical Field

[0001] The present invention belongs to the field of nuclear detection technology, specifically to the field of positron annihilation spectroscopy, and particularly relates to a β + -γ-γ coincides with the positron annihilation lifetime-momentum correlation (AMOC) spectrometer. Background Art

[0002] Positron annihilation spectroscopy is a unique method for characterizing material properties, especially for studying various types of defects, free volume and other microstructures in solid materials. Generally speaking, after a high-energy positron enters the interior of a material, it will undergo three main processes: thermalization, diffusion, and annihilation. Since positrons carry a positive charge, they will automatically search for negatively charged microstructures when moving in the material, and eventually annihilate in these places. The positron annihilation lifetime (the time from generation to annihilation as a photon) depends on the distribution of electron density in the material, and the distribution of electron density is determined by the microstructure in the material. Therefore, by detecting the lifetime of positrons in the material, the microstructural information in the material can be inferred. Based on this principle, a positron annihilation lifetime spectrometer was developed to measure the annihilation lifetime of positrons in matter.

[0003] In addition, when a positron annihilates with an electron in a substance, it usually releases a pair of gamma photons emitted approximately back-to-back, with an energy close to 0.511 MeV. However, the actual energy of this pair of gamma photons depends on the momentum of the positron-electron annihilation pair that annihilates. Since the positron usually loses almost all of its kinetic energy in the thermalization process when forming an annihilation pair, the momentum of the annihilation pair is mainly contributed by the electron involved in the annihilation. It can be seen that the actual energy of the gamma photon pair produced by annihilation carries the momentum information of the electron at the annihilation site. Based on this principle, a positron annihilation (coincidence) Doppler broadening spectrometer was developed to measure the energy of gamma photons produced by the annihilation of positrons in matter.

[0004] Based on the positron annihilation lifetime spectrometer and the positron annihilation Doppler broadening spectrometer, the AMOC spectrometer was developed by measuring the annihilation lifetime of the positron and the energy of the gamma photons produced by the annihilation, which is used to obtain the change of the electron momentum distribution with the positron annihilation lifetime. The AMOC spectrometer is mainly used to study the chemical environment and structure of pores in porous materials, the formation and behavior of positronium, etc., and it has irreplaceable advantages.

[0005] Early AMOC spectrometer data acquisition systems were based on complex electronic components, usually using the γ-γ-γ coincidence principle and using 22Na isotopes are used as positron sources. The AMOC spectrum can be obtained by counting the positron annihilation lifetimes and the corresponding annihilation γ photon energies in a large number of coincidence events. In order to obtain accurate test results, the total counts of each spectrum usually need to be greater than 10 million. However, the coincidence count rate of conventional AMOC spectrometers is usually low, and generally only a few coincidence events can be measured per second. It takes about 10 days to test a spectrum with a sufficient total count, and there is also a large room for improvement in the coincidence time resolution. In addition, for AMOC spectrometers based on the γ-γ-γ coincidence principle, the positron source needs to be clamped between two samples during testing. When two 7.5-micron-thick Kapton films are used to wrap the positron source, there is about 13% of source components in the positron annihilation lifetime-momentum correlation spectrum obtained in the test, and it cannot be deducted, which will affect the accuracy of the test results to a certain extent. In the prior art, there are 68 Ge isotopes as positron sources for β + -γ-γ is consistent with AMOC spectrometer, using PMT as a component of positron detector, and also using complex electronic plug-in. With the development of slow positron beam technology, beam-based β + -γ-γ coincides with the AMOC spectrometer, which significantly improves the detection efficiency of the starting signal by directly detecting positrons as the starting signal of the positron annihilation lifetime, and thus significantly improves the coincidence count rate. + -γ-γ coincidence AMOC spectrometers usually have poor coincidence time resolution and are very expensive to build and maintain, so they have not been widely used. Summary of the invention

[0006] In order to solve the above technical problems, the present invention provides a β + -γ-γ coincides with the positron annihilation lifetime-momentum correlation (AMOC) spectrometer. + -γ-γ conforms to the principle and uses a combination of silicon photomultiplier (SiPM) and scintillator as a positron detector; it uses a positron emission with lower energy 22 The Na isotope is used as a positron source, and the spectrometer is optimized by using a particle detection system simulation software to obtain spectrometer structural parameters suitable for this low-energy positron source; a semi-digital method combining an electronics plug-in and a fast digital oscilloscope is used to collect and process signals. The AMOC spectrometer of the present invention can effectively improve the coincidence counting rate, has good coincidence time resolution and extremely low source components, and is conducive to reducing the construction cost of the spectrometer.

[0007] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0008] A beta +-γ-γ conforms to the positron annihilation lifetime-momentum correlation spectrometer, including a positron annihilation lifetime detection module, a positron annihilation γ photon energy detection module, a signal acquisition module, and a control and analysis module, wherein:

[0009] The positron annihilation lifetime detection module is used to obtain the lifetime part of the AMOC spectrum;

[0010] The positron annihilation gamma photon energy detection module is used to obtain the momentum part of the AMOC spectrum by detecting the gamma photons generated by positron annihilation;

[0011] The signal acquisition module is used to collect the signals generated by the positron annihilation lifetime detection module and the positron annihilation gamma photon energy detection module;

[0012] The control and analysis module is used to control the signal acquisition module and obtain the AMOC spectrum based on the signal collected by the signal acquisition module.

[0013] The beneficial effects of the present invention are:

[0014] (1) The present invention uses a new type of SiPM to replace PMT as β + -γ-γ is a component of the positron detector of the AMOC spectrometer. Compared with PMT, SiPM has the characteristics of compact structure, high photon detection efficiency, and excellent time resolution. It is more suitable for obtaining the energy and time information of the signal under weak light and small luminous surface conditions, which is beneficial to improve the coincidence time resolution of the spectrometer. In addition, the price of SiPM is much lower than that of PMT, which is beneficial to reduce the construction cost of the spectrometer;

[0015] (2) The present invention uses a semi-digital data acquisition system to replace a purely digital data acquisition system based on complex electronic plug-ins, allowing the use of a fast digital oscilloscope with high vertical accuracy but low sampling rate to simultaneously complete the acquisition of positron annihilation lifetime and positron annihilation gamma photon energy signals. It further allows the use of various algorithms to perform pile-up judgment and de-pile-up processing on high-purity germanium signals, and allows the proportion of accidental coincidence events to be effectively reduced by analyzing the relative positions of the two signals in the time window, all of which are conducive to improving the accuracy of the test;

[0016] (3) Use of the present invention 22 Na isotope substitution 68 Ge isotopes are used as positron sources. By optimizing the spectrometer with the help of particle detection system simulation software, the spectrometer structure parameters suitable for this low-energy positron source are obtained. 68 Ge isotope, 22 The Na isotope has a longer half-life of about 68The half-life of Ge isotopes is 3.5 times longer, which helps the coincidence count rate of the spectrometer remain stable for a longer period of time. In addition, the low-energy positron source has lower requirements on sample thickness. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 The present invention is a β + - γ-γ conforms to the schematic diagram of the structure of the positron annihilation lifetime-momentum correlation spectrometer;

[0018] Figure 2 The positron annihilation Doppler broadening spectrum and positron annihilation lifetime spectrum of amorphous SiO2 measured by the present invention are as follows;

[0019] Figure 3 It is the positron annihilation lifetime-momentum correlation spectrum of amorphous SiO2 measured based on the present invention. DETAILED DESCRIPTION

[0020] The present application is further described below in conjunction with the accompanying drawings and embodiments.

[0021] like Figure 1 As shown, the specific technical solution of the present invention is as follows: a β + -γ-γ coincident semi-digital positron annihilation lifetime-momentum correlation spectrometer, including positron annihilation lifetime detection module, positron annihilation γ photon energy detection module, signal acquisition module, control and analysis module. Among them, the positron annihilation lifetime detection module and the positron annihilation γ photon energy detection module are respectively connected to the signal acquisition module, and the signal acquisition module is connected to the control and analysis module. Specifically:

[0022] Positron annihilation lifetime detection module: This module is used to obtain the lifetime part in the AMOC spectrum. It consists of three sub-modules: the start signal module, the stop signal module, and the electronics plug-in module. The start signal module and the stop signal module are respectively connected to the electronics plug-in module.

[0023] Starting signal module: This module directly detects the positrons generated by the decay of the positron source to obtain the starting signal of the positron annihilation lifetime, including SiPM and SiPM circuit board for signal readout, 22 NaCl solution, light-guiding film, first scintillator, in-phase amplifier, inverting amplifier, and constant-stable low-voltage power supply. Among them, the constant-stable low-voltage power supply is used to power the SiPM circuit board, and the light-guiding film can be scraped from a two-component optical adhesive. The simulation results obtained using the particle detection system simulation software Genat4 show that the optimal thicknesses of the light-guiding film and the first scintillator are 10 microns and 100 microns, respectively. The production process of the starting signal module is as follows: first, stick the light-guiding film (the surface of the newly prepared light-guiding film has a certain degree of stickiness) on the upper surface of the first scintillator; then, 22The NaCl solution was transferred to the center of the light-guiding film, and then transferred drop by drop and dried. The source spot size was controlled at about 2 mm. 22 After the NaCl solution is fully dried, apply a small amount of optical glue on the edge of the optical window of the SiPM and 22 The light-guiding film of the NaCl radiation source and the first scintillator are attached to the optical window of the SiPM with the light-guiding film as the contact surface. 22 NaCl radioactive source 22 Na isotope is used as a positron source. Some positrons produced by its decay will enter the first scintillator and deposit energy therein. When particles deposit energy in the first scintillator, the first scintillator will produce scintillation photons. When SiPM detects these scintillation photons, it will generate electrical signals, which will be amplified in turn by the in-phase amplifier and the inverting amplifier to obtain the starting signal of the positron annihilation lifetime.

[0024] Termination signal module: This module obtains the termination signal of the positron annihilation lifetime by detecting the gamma photons generated by positron annihilation. It includes PMT, a second scintillator (cylindrical, rectangular, etc.), reflective material (Teflon tape or other), black electrical tape, silicone grease, and the first constant high-voltage power supply to power the PMT. The production process of this module is as follows: Taking the cylinder as an example, first cover the upper bottom surface and side surfaces of the second scintillator with reflective material; then drop a small amount of silicone grease in the center of the optical window of the PMT, and stick the lower bottom surface of the second scintillator on the optical window of the PMT, and slowly rotate the second scintillator in the same direction for about 10 minutes to evenly distribute the silicone grease at the interface; finally, use black electrical tape to fix the second scintillator covered with reflective material on the optical window of the PMT, and further play a role in shielding from light. When placing the detector, make the lower cut surface of the second scintillator tangent to the upper surface of the first scintillator in the starting signal module (such as Figure 1 As shown in the figure, on the same dotted line extension line), it is beneficial to reduce the source component. In this module, when an annihilation γ photon deposits energy in the second scintillator, the second scintillator will generate scintillation photons, and when the PMT detects these scintillation photons, it will generate an electrical signal, thereby obtaining the termination signal of the positron annihilation lifetime.

[0025] Electronics plug-in module: This module is used to process the signals generated by the start signal module and the stop signal module, including two constant ratio timing discriminators (the first and second constant ratio timing discriminators), a delay device, and a time amplitude converter. Among them, the first constant ratio timing discriminator is used to extract the starting time of the positron annihilation lifetime from the signal output by the starting signal module by the constant ratio timing method, and the lower threshold of the energy window is slightly larger than the amplitude of the background signal. The upper threshold of the energy window can theoretically be set to include the entire energy spectrum of the starting signal end signal; the second constant ratio timing discriminator is used to extract the ending time of the positron annihilation lifetime from the signal output by the termination signal module by the constant ratio timing method, and the upper and lower thresholds of the energy window are set on both sides of the annihilation γ photon full energy peak; the delay device is connected to the back of the second constant ratio timing discriminator for processing the termination signal, and is used to add a certain time delay to the ending time of the positron annihilation lifetime; the time amplitude converter is used to receive the signals output by the first constant ratio timing discriminator and the delay device for processing the starting signal, and convert the time difference between the starting time and the delayed ending time of the positron annihilation lifetime into a pulse signal with an amplitude proportional to it, that is, the amplitude of the signal reflects the length of the positron annihilation lifetime.

[0026] Positron annihilation gamma photon energy detection module: This module obtains the momentum part of the AMOC spectrum by detecting the gamma photons generated by positron annihilation. It includes a high-purity germanium detector, a main amplifier, and a second constant-voltage power supply to power the high-purity germanium detector. When the gamma photons generated by positron annihilation enter the sensitive area of ​​the high-purity germanium detector and deposit energy, the high-purity germanium detector will generate a pulse signal, and the preamplifier inside the high-purity germanium detector and the external main amplifier will amplify the signal in turn. When placing the high-purity germanium detector, make the side of the high-purity germanium in the high-purity germanium detector tangent to the upper surface of the first scintillator in the starting signal module (such as Figure 1 As shown in the figure, on the same dotted line extension), it is beneficial to reduce the source component. The signal generated by this module will carry high-precision energy information of the annihilated γ photons, from which the momentum part of the AMOC spectrum can be calculated.

[0027] Signal acquisition module: This module is used to collect signals generated by the electronics plug-in module and the positron annihilation gamma photon energy detection module, and includes a fast digital oscilloscope. Among them, the fast digital oscilloscope should have the characteristics of high vertical accuracy and low sampling rate. The fast digital oscilloscope usually has four channels, A, B, C, and D. Channel A is used to collect the electrical pulse signal output by the time-amplitude converter, and channel C is used to collect the electrical pulse signal output by the main amplifier of the high-purity germanium detector. Channels B and D are not used. When the signal of channel A of the digital oscilloscope meets the trigger condition, the pulse signals of channels A and C within a certain time window are digitally processed and transmitted to the computer of the control and analysis module.

[0028] Control and analysis module: This module is used to control the signal acquisition module and obtain the AMOC spectrum from the acquired signals. It includes a computer and a C++ program, and consists of two sub-modules: the acquisition control module and the data analysis and recording module.

[0029] Acquisition control module: This module is used to control the signal acquisition module. Use C++ program to realize the interaction between the fast digital oscilloscope and the computer, and complete the real-time transmission of signals while controlling the fast digital oscilloscope.

[0030] Data analysis and recording module: This module is used to extract and record the AMOC spectrum from the collected signals. First, determine whether the signal of the C channel meets the threshold condition, that is, determine whether the amplitude of the signal of the channel falls within the full energy peak range of the annihilation γ photons in the energy spectrum at the end of the termination signal. If the threshold condition is not met, the current case is discarded, otherwise it is determined whether there is serious accumulation of the signal of the C channel. If there is serious accumulation, the current case is also discarded, otherwise it is determined whether the relative position of the signals of the A channel and the C channel in the time window corresponding to the current case meets the set relative position range, which will effectively reduce the proportion of accidental coincidence events. If the set relative position range is not met, the current case is also discarded, otherwise the baseline-corrected amplitude values ​​of the signals in the A channel and the C channel in the current case are recorded, that is, a set of positron annihilation lifetime-momentum data. Two-dimensional frequency statistics are performed on a large number of paired positron annihilation lifetime-momentum data to obtain the AMOC spectrum.

[0031] Example

[0032] according to Figure 1 The AMOC spectrometer involved in the present invention was actually built, and preliminary experimental results were obtained. When the spectrometer was built, the first scintillator used a 100-micron plastic scintillator EJ-228, the 10-micron light-guiding film was made of SL600 optical glue, the second scintillator used a cylindrical BaF2, the fast digital oscilloscope used a PicoScope 5444D, the SiPM used onsemi's MICROFJ-60035, the PMT used Hamamatsu H6610, the in-phase amplifier used ZFL-1000LN, and the inverting amplifier used OPA855. 22 The Na positron source intensity is about 40 microcuries, and the distance between the high-purity germanium detector and the positron source is about 45 cm. Using the AMOC spectrometer built, the amorphous SiO2 sample was tested. The positron annihilation Doppler broadening spectrum and positron annihilation lifetime spectrum of the test results are as follows: Figure 2 The corresponding AMOC spectrum (only about 450,000 total counts were measured in the preliminary experiment) is shown in Figure 3The decomposition results of the positron annihilation lifetime spectrum and the St curve calculated from the AMOC spectrum are basically consistent with the results reported in the literature, indicating that the AMOC spectrometer of the present invention can achieve the expected experimental goals.

[0033] The specific embodiments described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A beta + -γ-γ coincident positron annihilation lifetime-momentum correlation spectrometer, characterized in that It includes positron annihilation lifetime detection module, positron annihilation γ photon energy detection module, signal acquisition module, control and analysis module, among which, The positron annihilation lifetime detection module is used to obtain the lifetime part of the AMOC spectrum; The positron annihilation gamma photon energy detection module is used to obtain the momentum part of the AMOC spectrum by detecting the gamma photons generated by positron annihilation; The signal acquisition module is used to collect the signals generated by the positron annihilation lifetime detection module and the positron annihilation gamma photon energy detection module; The control and analysis module is used to control the signal acquisition module and obtain the AMOC spectrum based on the signal collected by the signal acquisition module.

2. A β according to claim 1 + -γ-γ coincident positron annihilation lifetime-momentum correlation spectrometer, characterized in that The positron annihilation lifetime detection module comprises a start signal module, a stop signal module and an electronics plug-in module, wherein the start signal module and the stop signal module are respectively connected to the electronics plug-in module.

3. A β according to claim 2 + -γ-γ coincident positron annihilation lifetime-momentum correlation spectrometer, characterized in that The starting signal module is used to obtain the starting signal of the positron annihilation lifetime, including SiPM and a SiPM circuit board for signal readout, 22 NaCl solution, light-guiding film, first scintillator, in-phase amplifier, inverting amplifier, constant-voltage power supply, wherein: The first scintillator, 22 The light-guiding film of the NaCl solution and the SiPM are sequentially stacked on the SiPM circuit board; The constant low voltage power supply is used to supply power to the SiPM circuit board; Said 22 NaCl solution is used to provide 22 The Na isotope is used as a positron source. When some positrons generated by the decay of the positron source enter the first scintillator and deposit energy, the first scintillator generates scintillation photons. The SiPM is used to detect scintillation photons and generate electrical signals, which are amplified in sequence by a common-phase amplifier and an inverting amplifier to obtain a starting signal of the positron annihilation lifetime.

4. A β according to claim 2 + -γ-γ coincident positron annihilation lifetime-momentum correlation spectrometer, characterized in that The termination signal module obtains the termination signal of the positron annihilation lifetime by detecting the gamma photons generated by the positron annihilation, and includes a PMT, a second scintillator, and a first constant high-voltage power supply for powering the PMT, wherein the second scintillator is covered with a reflective material and fixed to the optical window of the PMT, and the lower section of the second scintillator is tangent to the upper surface of the first scintillator in the start signal module, and when an annihilation gamma photon deposits energy in the second scintillator, the second scintillator generates scintillation photons, and when the PMT detects the scintillation photons, it generates an electrical signal, i.e., the termination signal of the positron annihilation lifetime.

5. A β according to claim 2 + -γ-γ coincident positron annihilation lifetime-momentum correlation spectrometer, characterized in that The electronics plug-in module is used to process the start signal and the end signal generated by the start signal module and the end signal module, and includes a first and a second constant ratio timing discriminator, a delay device, and a time-amplitude converter, wherein the first constant ratio timing discriminator is used to extract the start time of the positron annihilation life from the start signal output by the start signal module by a constant ratio timing method, and the second constant ratio timing discriminator is used to extract the end time of the positron annihilation life from the end signal output by the end signal module by a constant ratio timing method. The delay device is connected to the second constant ratio timing discriminator and is used to add a certain time delay to the end time of the positron annihilation life. The time-amplitude converter is used to receive the signals output by the first constant ratio timing discriminator and the delay device, and convert the time difference between the start time and the delayed end time of the positron annihilation life into a pulse signal.

6. A β according to claim 1 + -γ-γ coincident positron annihilation lifetime-momentum correlation spectrometer, characterized in that The positron annihilation gamma photon energy detection module includes a high-purity germanium detector, a main amplifier, and a second constant-voltage power supply for powering the high-purity germanium detector. When the gamma photons generated by positron annihilation enter the sensitive area of ​​the high-purity germanium detector and deposit energy, the high-purity germanium detector generates a pulse signal, and the pulse signal is amplified by an external main amplifier to calculate the momentum part in the AMOC spectrum.

7. A β according to claim 1 + -γ-γ coincident positron annihilation lifetime-momentum correlation spectrometer, characterized in that The signal acquisition module includes a fast digital oscilloscope, which includes four channels, one channel is used to acquire the pulse signal output by the time amplitude converter, and one channel is used to acquire the pulse signal output by the main amplifier connected to the high-purity germanium detector.

8. A β according to claim 1 + -γ-γ coincident positron annihilation lifetime-momentum correlation spectrometer, characterized in that The control and analysis module includes an acquisition control module and a data analysis and recording module.

9. A β according to claim 8 + -γ-γ coincident positron annihilation lifetime-momentum correlation spectrometer, characterized in that The acquisition control module is used to control the signal acquisition module, and uses a C++ program to realize the interaction between the fast digital oscilloscope and the computer, and completes the real-time transmission of the signal while controlling the fast digital oscilloscope.

10. A β according to claim 8 + -γ-γ coincident positron annihilation lifetime-momentum correlation spectrometer, characterized in that The data analysis and recording module is used to extract and record the AMOC spectrum from the signal collected by the signal collection module.

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